1. Field of the Invention
This invention relates to improvements in the thermomagnetic recording technology and more in particular to a method and apparatus for recording information on a thermomagnetic recording medium with the use of its Curie point or compensation point.
2. Description of the Prior Art
In order to increase recording density in magnetic recording, the size of a bit pattern which constitutes an element in recording information on a recording medium must be made smaller. It is a well established concept that as a representative size .lambda. of the bit pattern is made smaller, the gap .delta. between a magnetic head and the magnetic recording medium must be made smaller proportionately. This is because, the magnetic force lines from a small magnet extend further in proportion to its size. It has been known that the following approximate relation holds between representative size .lambda. of the bit pattern, gap .delta. between the magnetic head and the recording medium, and output signal level D (in decibels) when reproduced by the magnetic head. EQU D.perspectiveto.-55(.delta./.lambda.)
As is obvious from the above equation, when the gap .delta. between the magnetic head and the magnetic recording medium has a finite value, the level D of the output signal read by the magnetic head decreases by certain decibels determined in accordance with the above equation as compared with the reference output signal level when the gap .delta. is zero, i.e., the magnetic head being in contact with the recording medium. For example, suppose that the representative size .lambda. such as bit pattern spacing is 1 .mu.m, then even if the gap .delta. between the magnetic head and the recording medium is as small as 0.4 .mu.m, the decrease in signal level amounts to about 22 decibels, indicating a significant decrease in output signal level. Thus, in order not to decrease the level of output signal to an unacceptable degree, the value of .delta./.lambda. must be maintained approximately in the range between 0.2 and 0.4.
FIG. 1 shows the typical arrangement of a prior art vertical magnetic recording system. As shown, a permalloy strip 1 is enclosed by a protective material 2 such as plastics to form a main magnetic pole 3. Opposite to the pole 3 is disposed a counter magnetic pole 5 comprised of a ferrite body having a high permeability and wound around by a coil 4. The main and counter magnetic poles 3 and 5 together form a magnetic head. A vertical magnetic recording medium M is inserted between the main and counter magnetic poles 3 and 5, thereby information may be recorded onto or read out of the medium M. The medium M may be constructed by forming on the surface of a substrate an amorphous magnetic thin film comprised of a family of rare earth-transition metal elements such as Co-Cr, Mn-Bi, Tb-Fe, Gd-Fe, Dy-Fe, etc., by means of vapor deposition, sputtering and the like.
FIG. 2 shows another prior art recording system for carrying out high density magnetic recording similarly with the system of FIG. 1. The system of FIG. 2 is often referred to as an optomagnetic recording system because use is made of a laser beam as means for applying heat to the recording medium M for recording information. As shown, a laser beam carrying therein information to be recorded is emitted from a recording laser device 6 and caused to impinge upon the medium M through a convergent lens 7. The portion of the medium M where the beam impinges upon becomes locally heated, and when the temperature of the heated portion rises above the Curie point of the vertical magnetic recording medium M, its local coercive force sharply decreases. Under the condition, the direction of magnetization of the thus heated portion is reversed due to an external magnetic field applied by a coil 8, thereby recording information on the medium M.
It should however be noted that the prior art recording system of FIG. 1 is disadvantageous because it is extremely difficult to maintain a proper gap between the magnetic head and the vertical magnetic recording medium M. Besides, the magnetic head suffers from wear due to occasional contact with the medium M. The prior art system of FIG. 2 is also disadvantageous since its recording density is rather limited. That is,in this case, the characteristic size of a bit to be recorded cannot be made smaller than the wavelength of the laser beam because of diffraction and other phenomena.